Animals in the phylum Platyhelminthes are called flatworms because they are flattened from head to tail. Flatworms share several features with more derived animal phyla. They are the most primitive group to exhibit bilateral symmetry. Flatworms have three embryonic tissue layers: ectoderm, mesoderm, and endoderm.

Animals within Platyhelminthes show more complexity than ancestral phyla, but are not as complex as more derived animal phyla. They are acoelomates, which means they do not have a body cavity. Platyhelminthes are unsegmented. They have muscles and a simple nervous system that includes a primitive brainlike structure which is formed from a thickening of the ventral nerve cords in the head region. They have a mouth, but no anus, and a primitive digestive cavity. They also have a primitive excretory system. They do not have a respiratory or circulatory system and are limited to simple diffusion for gas exchange. They can regenerate by fission as well as reproduce sexually, sometimes with complex life cycles passing through more than one host. Flatworms move about using cilia and by undulating movements of the whole body.

Almost all Platyhelminthes are aquatic, both fresh water and marine, but a few terrestrial species live in moist, warm areas. Species vary in size from microscopic to over 60 feet (20 meters) long for some tapeworms.

There are four major classes of Platyhelminthes and over twenty-five thousand species. Flatworms in the class Turbellaria are marine and freshwater free-living scavengers. The other three classes are parasitic and include some of the most harmful human parasites. The classes Trematoda, commonly called flukes, and Momogea are both endoparasites and ectoparasites. Momogea are parasites of aquatic vertebrates such as fish. Flatworms in the class Cestoda are endoparasites known as tapeworms.

See Also

Phylogenetic Relationships of Major Groups

Bibliography

Anderson, D. T., ed. Invertebrate Zoology. Oxford: Oxford University Press, 1998.

Barnes, Robert D. Invertebrate Zoology, 5th ed. New York: Saunders College Publishing, 1987.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life the Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.